Weeds are a tremendous problem for farmers and growers throughout the world. It has been estimated that about 41% of the cost of plant protection is for the control of weeds. Herbicides are applied to more acres than fungicides and insecticides combined. Weeds act as alternate hosts for insects, fungi, bacteria, and viruses. They affect man, not only by competing with crop plants, but by poisoning range animals, interfering with right-of-ways and roadways, decreasing forest production, and marring landscapes.
Weeds compete with crops for moisture, light, and nutrients and can inhibit crop growth and reduce yields. Competition can be particularly detrimental in tree crops when young trees are being established. In citrus and other tree crops the most problematic weeds are some annual and perennial grass weeds, which are also among the world's worst weeds in many crops worldwide. Some notable examples are: large crabgrass (Digitaria sanguinalis [L.] Scop.), crowfootgrass (Dactyloctenium aegyptium [L.] Willd.), johnsongrass (Sorghum halepense [L.] Pers.), bermudagrass (Cynodon dactylon [L.] Pers.), guineagrass (Panicum maximum Jacq.), southern sandbur (Cenchrus echinatus L.), Texas panicum (Panicum texanum Buckl.), and yellow foxtail (Setaria glauca [L.] Beauv.).
The use of chemical pesticides in agriculture is currently a major concern in the U.S. Because herbicides are so widely used in agriculture, and because they are often applied directly to the soil, the potential for their movement into groundwater by leaching is perhaps greater than any other pesticide. Other inadequacies of chemical controls include lack of residual control, injury to non-target organisms, undesirable residues in harvested products, and carryover in subsequent crops.
Therefore, the use of bioherbicides is becoming an increasingly important alternative to chemical herbicides. This importance is exemplified by several patents which have been issued for bioherbicides and their use. Some of these patents, by way of illustration, are as follows: U.S. Pat. No. 3,849,104 (control of northern jointvetch with Colletotrichum gloeosporioides Penz. aeschynomene); U.S. Pat. No. 3,999,973 (control of prickly sida [teaweed] and other weeds with Colletotrichum malvarum); U.S. Pat. No. 4,162,912 (control of milkweed vine with Araujia mosaic virus); U.S. Pat. No. 4,626,271 (cyanobacterin herbicide); U.S. Pat. No. 4,915,726 (biological control of dodder); U.S. Pat. No. 5,256,627 (control of nutsedge using Curvularia and Fusarium fungi).
The bioherbicide technology involves mass production, formulation, and inundative application of highly virulent, host-specific plant pathogenic microorganisms at a time conducive for disease development on a population of weeds. This technology has wide commercial applications in weed management. The major challenges to the bioherbicide technology are host-specificity (which typically limits use to one weed per pathogen) and inadequate weed control.